Tombstone, as far as I know. If you check out their FAQs on the website, I think you'll find that what these guys did was out of step with the project's intentions:
No "hostile" confrontation with intruders will be allowed. The MMProject participants will only "spot and report", the USBP takes it from there.
We will seal the border by our presence, but will not violate anyone's civil rights, and will not abuse anyone from any country.
http://www.minutemanproject.com/FAQ.html
No confrontations whatsoever. MMP participants are supposed to remove themselves from any area where illegal aliens show up. They must keep their distance and call Border Patrol. They are not supposed to attempt to stop anyone and just let them move on if they keep heading North. They are supposed to document, with video cameras, all encounters to make sure that no one can invent charges against the MMP volunteers.